The present invention generally pertains to methods of protection or switching data signals being transmitted through optical data networks via optical conductors.
In existing optical data networks, optical data signals with a low bit rate are multiplexed to form data signals with a higher bit rate and are transmitted via the optical data networks. These optical data networks include various optical amplifiers and switching devices.
In order to protect line sections of the optical data networks against failures, stand-by optical conductors, also termed protection lines, are laid in parallel with the operational optical conductors, also termed working lines.
These disjoint, stand-by optical conductors or links protect the operational data signals. Given the presence of more than two disjoint paths, it is economically advantageous when n operational data signals share the alternative route. This prevents 50% of the total transport capacity being reserved for stand-by switchings. Only 1/(n+1)th of the transport capacity has to be reserved for stand-by switchings, owing to the 1:n stand-by switching.
Protection switchings, also termed protection, for optical transmission systems are known from the synchronous digital hierarchy, SDH for short, or the synchronous optical network, SONET for short. A known protection switching is the 1:n or, for the case n=1, the 1:1 stand-by switching. In this case, n optical lines are protected by one stand-by line. Additional data that are no longer transmitted in the event of a fault can be transmitted in the fault-free state on the stand-by line. Data transmitted additionally on the stand-by line are also denoted as low-priority traffic.
A 1:n stand-by switching method, termed 1:n multiplex section protection in accordance with ITU-T G.783, has been standardized for this purpose in SDH technology. What is termed a multiplex section is used in SDH for the purpose of unambiguous fault location. This section forms a multiplex section overhead over the operational data signal and extends precisely over the section of the stand-by switching. In other words, the multiplex section overhead is generated at the transmitting end and terminated at the receiving end, or precisely the reverse in the opposite direction. The multiplex section overhead is therefore not affected by operational data signal interference outside the stand-by switching system. If, for example, interference is determined on the multiplex section at the receiving end, the cause of the interference clearly resides on the section of the multiplex section, and not before it. The same monitoring principle also holds for the alternative route. The multiplex section overhead, which is transmitted with the useful data signal, makes a fast communication channel available for coordinating the stand-by switching. This ensures that both ends are nevertheless switched to stand-by in the event of interference in only one direction, that both ends switch the same operational data signal to stand-by in accordance with a priority scheme in the event of the occurrence of multiple faults, and that the two ends are reset in a quasi-synchronous fashion after the fault clearance.
An automatic protection switching protocol, termed APS for short, is normally used to control this 1:n stand-by switching. This protocol is transmitted in overhead bytes of the optical data signals, for example, what is termed the multiplex section overhead. The protocol overhead may be transmitted overhead with the aid of additional optical channels, such as optical supervisory channels, OSC for short, for example. The protocol is evaluated at the initial point and end point of the transmission. Protection switching is then possible, if necessary, after evaluation of the protocol data.
DWDM network operators offer the SDH or SONET network operators transparent links on the basis of dense wavelength division multiplex technology, DWDM for short. The transparency relates to the SDH or SONET signals, that is to say to payload and overload, thus including the multiplex section overhead. The DWDM network operator can certainly read the multiplex section overhead and thus monitor the signal quality. However, owing to the business model, the network operator is not authorized to modify or terminate the multiplex section overhead. The advantage of this for the SDH network operator is that the operator can use the multiplex section overhead to communicate between his network segments over the DWDM network.
The effects of this on the 1:n stand-by switching are as follows. The multiplex section no longer extends only over the section between two devices to be switched to stand-by (DWDM network operators). It certainly still supplies a criterion for the failure of an operational data signal. However, the fault cause can no longer be located. Because the multiplex section overhead can no longer be used by the DWDM network operator, no communication channel is available for coordinating the 1:n stand-by switching.
Accordingly, there is now no optical additional channel available, or it is not possible to access overhead bytes of the optical data signal. This problem as can also occur in the case of the transmission of data over networks having devices from different manufacturers or over externally administered networks. There is the problem that it is not possible to carry out protection switching with the aid of these APS protocols.